Canvas 对象未显示，但位置在 Java Applet 中正确更新

我的基本弹跳球小程序应该有一个球根据中的值移动loc变量，但什么也没有显示。打印输出loc表明移动它和弹离边界背后的数字/数学确实按其应有的方式工作，但没有任何显示。查看在线示例，我不明白为什么我的绘图/绘画代码没有按预期工作。这就是我所拥有的，问题可能集中在中间位置Ball对象是。我发布了程序的大部分内容，因为如果有人运行它，它就会编译/运行。

import java.awt.*;
import java.applet.*;
import java.awt.event.*;

public class Bounce2 extends Applet implements ActionListener, AdjustmentListener, Runnable
{
    //runtime variables
    boolean running = false;
    boolean kill = false;

    //buttons
    Button runbutton = new Button("Run"); 
    Button pausebutton = new Button("Pause");
    Button quitbutton = new Button("Quit");

    //text
    Label speedlabel = new Label("Speed");
    Label sizelabel = new Label("Size");

    //scrollbars
    private final int barHeight = 20, SLIDER_WIDTH = 10, MAXSPEED = 110, MINSPEED = 0, MAX_SIZE = 110, MIN_SIZE = 10;
    Scrollbar speedbar = new Scrollbar(Scrollbar.HORIZONTAL, MAXSPEED/2, SLIDER_WIDTH, MINSPEED, MAXSPEED);
    Scrollbar sizebar = new Scrollbar(Scrollbar.HORIZONTAL, MAX_SIZE/2, SLIDER_WIDTH, MIN_SIZE, MAX_SIZE);

    //drawn objs
    Ball ball;
    int size = 50;  
    private Graphics obj;
    Image offscreen = null;
    Point loc = new Point(100,100); //location of the ball
    private Thread ballThread;

    //boundaries
    int boundx = 640;
    int boundy = 400;

    //directions
    int dx = 1; //1 = left, -1 = right
    int dy = 1; //1 = up, -1 = down

    //speed
    int speed = speedbar.getValue();
    int delay;

    //initialize the applet and draw everything
    public void init()
    {
        double colWeight[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
        double rowWeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
        int colWidth[] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};//15 cols
        int rowHeight[] = {1,1,1,1,1,1,1,1,1,1}; //10 rows
        GridBagConstraints c = new GridBagConstraints();
        GridBagLayout gbl = new GridBagLayout();
        gbl.rowHeights = rowHeight;
        gbl.rowWeights = rowWeight;
        gbl.columnWeights = colWeight;
        gbl.columnWidths = colWidth;
        c.anchor = GridBagConstraints.CENTER;

        setBounds(0,0,480,640);
        setLayout(new BorderLayout());
        Panel controlpanel = new Panel();
        controlpanel.setLayout(gbl);
        controlpanel.setSize(640,80);

        Panel drawingpanel = new Panel(null);
        drawingpanel.setSize(640,400);
        ball = new Ball();
        drawingpanel.add(ball);
        drawingpanel.setVisible(true);

        //speed scrollbar
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 1;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.speedbar,c);

        //run button
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 2;
        c.gridheight = 1;
        c.gridx = 5;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.runbutton,c);

        //pause button
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 2;
        c.gridheight = 1;
        c.gridx = 8;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.pausebutton,c);

        //size scrollbar
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 11;
        c.gridy = 7;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.sizebar,c);

        //speed text label
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 1;
        c.gridy = 8;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.speedlabel,c);

        //size text label
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 11;
        c.gridy = 8;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.sizelabel,c);

        //quit button
        c.weightx = 1;
        c.weighty = 1;
        c.gridwidth = 3;
        c.gridheight = 1;
        c.gridx = 6;
        c.gridy = 9;
        c.fill= GridBagConstraints.HORIZONTAL;
        gbl.setConstraints(this.quitbutton,c);

        //add to the screen
        controlpanel.add(this.speedbar);
        controlpanel.add(this.runbutton);
        controlpanel.add(this.pausebutton);
        controlpanel.add(this.sizebar);
        controlpanel.add(this.speedlabel);
        controlpanel.add(this.sizelabel);
        controlpanel.add(this.quitbutton);

        //add listners
        speedbar.addAdjustmentListener(this);
        runbutton.addActionListener(this);
        pausebutton.addActionListener(this);
        sizebar.addAdjustmentListener(this);
        quitbutton.addActionListener(this);

        //add the panels
        add("South", controlpanel);
        add("Center", drawingpanel);

        //drawing paramaters
        obj = drawingpanel.getGraphics();
        loc = new Point(loc.x+dx, loc.y+dy);

        setVisible(true);
        validate();
    }

    public void start()
    {
        if (ballThread == null)
        {
            ballThread = new Thread(this);
            ballThread.start();
            repaint();
        }
    }

    public void run()
    {
        Thread.currentThread().setPriority(Thread.MAX_PRIORITY);
        while (!kill)
        {
            if (running)
            {
                repaint();
            }
            try
            {
                Thread.sleep(delay);
            }
            catch(InterruptedException e){System.err.println("Interrupted.");}
        }
        stop();
    }

    //class to handle animations
    class Ball extends Canvas 
    {
        public void move()
        {
            //if it will hit the right or left, flip the x direction and set it 
            if (loc.x+size >= boundx || loc.x <= 0)
            { dx *= -1; }
            loc.x += dx;
            //if it will hit the top or bottom, flip the y direction and set it
            if (loc.y+size >= boundy || loc.y <= 0)
            { dy *= -1; }
            loc.y += dy;
            setBounds(dx,dy,size,size);
        }   

        public void paint(Graphics g)
        {
            update(g);
        }

        public void update(Graphics g)
        {
            Graphics buffer;
            if (offscreen == null);
            {
                offscreen = createImage(boundx, boundy);
                buffer = offscreen.getGraphics();
            }

            buffer.setColor(getBackground());
            buffer.fillRect(loc.x,loc.y,boundx, boundy);
            //update loc
            move();

            //draw
            buffer.setColor(Color.black);
            buffer.drawOval(loc.x, loc.y, size, size);
            buffer.fillOval(loc.x, loc.y, size, size);

            //draw rectangles out of vector 
            super.paint(g);
            g.drawImage(offscreen, loc.x, loc.y, null);
        }
    }

    public void actionPerformed(ActionEvent e)
    {
        Object source = e.getSource();
        if (source == this.runbutton)
        {
            running = true;
        }
        else if (source == this.pausebutton)
        {
            running = false;
        }
        else if (source == this.quitbutton)
        {
            //kill processes
            kill = true;
            //remove listeners
            stop();
        }
    }

    public void adjustmentValueChanged(AdjustmentEvent e)
    {
        Object source = e.getSource();
        //set the new size. 
        if (source == sizebar)
        {
            //check for clipping
            int newsize = sizebar.getValue();

            // x
            if (loc.x+newsize >= boundx)
            { 
                newsize = boundx - loc.x - 1;
                sizebar.setValue(newsize);
            }

            // y
            if (loc.y+newsize >= boundy + 100)
            {
                newsize = boundy+100 - loc.y - 1;
                sizebar.setValue(newsize);
            }
            size = newsize;
        }
        if (source == speedbar)
        {
            speed = speedbar.getValue();
            delay = MAXSPEED - speedbar.getValue();
        }
    }

    public void stop()
    {
        this.speedbar.removeAdjustmentListener(this);
        this.runbutton.removeActionListener(this);
        this.pausebutton.removeActionListener(this);
        this.sizebar.removeAdjustmentListener(this);
        this.quitbutton.removeActionListener(this);
        Thread.currentThread().setPriority(Thread.MIN_PRIORITY);
    }
}

我对您的代码进行了以下更改，并且可以使其正常工作 -

首先我将球的初始位置设置为

Point loc = new Point(10,10);

初始化后ball = new Ball(); added -

drawingpanel.setLayout(new BorderLayout());
            drawingpanel.add("Center",ball);

在 run() 方法中重新绘制球 -

if(running){
ball.repaint(); 
}

在 move() 方法中删除/注释该行 -

//setBounds(dx,dy,size,size);

在 update(Graphics g) 方法中删除/注释行 -

//buffer.setColor(Color.black);
//buffer.fillRect(loc.x,loc.y,boundx, boundy);

这 ; if (offscreen == null) 之后；没有任何意义，所以我评论它 -

 //if (offscreen == null); 

宾果，一切正常！如果您能够成功，请告诉我，否则我将发布更新的代码。